winit 0.30 reports no system theme there (EventLoop::system_theme is
None), so the "system" preference resolved to light every time. When
the windowing stack gives no theme, the interface now follows the
freedesktop colour scheme instead: read once and watched live through
the settings portal (org.freedesktop.appearance / color-scheme) in the
desktop services (Services::system_dark), so GNOME's and KDE Plasma 6's
appearance setting switches the app without a restart. Where the OS
theme is reported (macOS, Windows, web) it still wins, and without any
answer the light fallback stands.